Air brake systems for railway cars generally require an air reservoir or tank for supplying air under pressure to the brake components for actuating the latter between operative and inoperative positions. Heretofore such air reservoirs have been constructed as a separate unit and attached on the underframe of the railway car. These prior art reservoirs normally include an auxiliary pressure compartment and an emergency pressure compartment. The resulting structure of these self-contained air brake air reservoirs required considerable space on the underframe for mounting thereon. At the same time the reservoirs structure added to the overall weight of the car and also expense to manufacture.